I'm not 100% sure this is right, but I wanted to disable them too, because of some bug which makes forts/towers forever unbuildable in regions hit. Anyway, here's what I did and it seems to have worked, no buggy disasters since:

Find a file called descr_disasters in the data\world\maps\base directory.
Open it with notepad.
Put a ; before each of the lines of the disaster you want stopped. Basically tells the game to ignore that line. I guess deleting them would do the same thing but I wasn't sure how the game would react to the lines being removed and wanted to be able to undo the changes easily.
Save and play, haven't had a problem, or a disaster (I did leave Horde and Storm active since they don't seem to cause any glitches that I'm aware of).

Again, I'm not a regular modder of this game and thus not 100% on anything, but it's been working fine for me for several weeks of daily play. I'm not sure how the game would react to a set event (with a set date) from the descr_events file requesting the disaster information but again, hasn't seemed to cause any problems for me.

Note: File may have to have the "Read-Only" option unselected in its properties to save.

Hope it helps.